1. Field of the Invention
This invention relates to a method for forming holes in a silicon substrate. More particularly, the invention relates to a method, for forming holes in a silicon substrate, that irradiates a silicon substrate having a pattern formed on one of the surfaces thereof with a laser beam, and forms through-holes or recesses.
2. Description of the Related Art
The performance of electronic devices has become higher and their sizes have become smaller in recent years. Along with this trend, a so-called xe2x80x9cmulti-chip package (MCP)xe2x80x9d as a semiconductor device in which a plurality of semiconductor chips is arranged in one package has been fabricated to improve the device performance and to reduce the device size. A three-dimensional MCP, shown in FIG. 7, in which a plurality of semiconductor chips is stacked in a direction of thickness, in particular, exhibits a greater contribution to miniaturization of electronic devices than a two-dimensional MCP in which a plurality of semiconductor chips is arranged in a planar direction, and has therefore been investigated vigorously (see, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 2001-94039 and 10-163411).
In such a three-dimensional MCP, the semiconductor chips so stacked must be electrically connected to one another. To this end, a member called a xe2x80x9cviaxe2x80x9d 102 is used to electrically connect the stacked semiconductor chips 100 with one another as shown in FIG. 7, which is more advantageous from the aspects of integration efficiency and mounting efficiency than the use of wires for electrically connecting the semiconductor chips 100 with one another. To electrically and mutually connect the semiconductor chips 100 through the via 102, as shown in FIG. 7, a through-hole 104 must be formed in the semiconductor chips 100. A method using plasma processing or a method using laser processing has been employed in the past to form a hole or holes, such as a through-hole for forming the via, in a substrate formed substantially of silicon used for the semiconductor chip 100.
Of these hole formation methods, the method using plasma processing can form a clean hole in the silicon substrate with extremely small amounts of debris adhering to an inner wall of a hole such as the through-hole. However, the hole formation method using plasma process is not free from the high cost of forming a hole such as the through-hole, and a hole formation method capable of forming a hole in a silicon substrate at a lower cost has therefore been required.
The method of forming a hole in the silicon substrate by laser processing can form a hole such as the through-hole in the silicon substrate at a lower cost compared to the method using plasma processing but cannot form a clean hole. When laser beams 204 are irradiated to a silicon substrate 200 that is held by a vacuum onto a stage 202 in the hole formation method using laser processing as shown in FIG. 8, a recess 206a formed in the silicon substrate 200 becomes gradually deeper and at last a through-hole 206 is formed in the silicon substrate 200. A deposit (laser dross) 208, as debris scattered from the silicon substrate 200 due to the irradiation of the laser beam 204 and deposited, exists at a portion in the proximity of the open periphery of the through-hole 206 thus formed on the irradiation side of the laser beam 204. In addition, the debris adheres to the inner wall of the through-hole 206 and to the surface of the silicon substrate facing the stage 202, as shown in FIG. 9.
When the laser dross 208 and the debris are left as such, they peel off the silicon substrate 200 in subsequent process steps, turn into foreign matter and are likely to invite a drop in reliability of the product. The laser dross 208 and the debris can be removed when the silicon substrate 200 is washed in a caustic potash (KOH) solution or the like, but a conductor pattern 210 formed on one side of the silicon substrate 200 is likely to be damaged during washing with the caustic potash solution. Also, when the laser beam forms the through-hole 206, heat is accumulated inside the silicon substrate 200 during the formation of the through-hole 206 and the conductor pattern 210 is likely to be thermally damaged. Therefore, a method of forming the holes in the silicon substrate using laser processing has not been industrially employed in the past.
However, in comparison with the hole formation method using plasma processing, the method of forming the holes in the substrate by laser processing can form the holes such as the through-hole at a lower cost. Therefore, if the heat dissipation property of the silicon substrate 200 can be improved during laser processing and if the laser dross 208 formed around the open periphery of the resulting hole such as the through-hole and the debris deposited on the inner wall of the hole can be easily cleaned, the hole formation method using laser processing could be advantageously used industrially.
It is therefore an object of the invention to provide a hole formation method of a silicon substrate by laser processing that can solve the problems described above, can improve heat dissipation property during processing and can easily clean the dross formed around an open periphery of the resulting holes, such as through-holes, and the debris deposited on the inner wall surface of the holes.
To solve the problems described above, the inventor first formed a protective film made of silicon oxide (SiO2) stable to a caustic potash solution on a conductor pattern-formed surface of a silicon substrate, then formed a nickel (Ni) plating film on the entire surface of the silicon substrate inclusive of the upper surface of this protective film, and then irradiated a laser beam to thereby form through-holes. As a result, the inventor has found that possible damage of the conductor pattern by heat of laser processing can be eliminated, the through-holes formed by laser processing can be easily cleaned, and clean through-holes can be formed. The inventor has thus completed the invention.
According to an aspect of the invention, there is provided a method of forming a through-hole or a recess in a silicon substrate having a conductor pattern formed on one side thereof, by irradiating laser beam to the silicon substrate, comprising the steps of forming a protective film for protecting the conductor pattern on the one side (pattern formation side) of the silicon substrate, forming, on the entire surface of the silicon substrate inclusive of top of the protective film, a metal plating film adhered to the protective film, irradiating a laser beam to a predetermined position of the silicon substrate covered with the protective film and with the metal plating film, to form a through-hole or a recess in the silicon substrate, peeling the metal plating film and removing a deposit deposited on the metal plating film around the open periphery of the through-hole or the recess during the formation thereof by the laser beam irradiation, and removing a deposit, such as debris, deposited on the inner wall of the through-hole or the recess during the formation thereof by the laser beam irradiation, by use of a removing solution not damaging the protective film.
In the invention described above, a silicon oxide (SiO2) film can be formed as the protective film, so as to allow it to be used also as an insulating film. When the nickel (Ni) plating film is formed as the metal plating film, adhesion with the silicon oxide (SiO2) film as the protective film can be improved.
When a caustic potash (KOH) solution is used as the removing solution for removing the deposit on the inner wall of the through-hole or the recess, the deposit can be easily removed. A solution of NaOH or HF can also be used besides the KOH solution.
In the case where the laser beam is irradiated to serially form a plurality of through-holes or recesses in the silicon substrate, following the formation of a previous through-hole or recess, a subsequent through-hole or recess may be formed at a location on the substrate, the location being selected so that heat generated by the irradiation of the laser beam to the substrate for the formation of the previous through-hole or recess is not substantially accumulated in the substrate during the formation of the subsequent through-hole or recess. Alternatively, also following the formation of a previous through-hole or recess, a subsequent through-hole or recess may be formed at a time interval between the formations of the previous and subsequent through-holes or recesses, the time interval being selected so that heat generated by the irradiation of the laser beam onto the substrate for the formation of the previous through-hole or recess is not substantially accumulated in the substrate during the formation of the subsequent through-hole or recess. In this way, accumulation of heat in the silicon substrate due to the irradiation of laser beam can be prevented.
Also in the invention described above, the debris formed by the irradiation of the laser beam is reflected by the surface of the stage and is deposited on the silicon substrate at the point when the silicon substrate is put on the stage, and the laser beam is irradiated thereto and penetrates through the silicon substrate to create the through-hole. Using a stage having debris reflection-prevention means, however, it becomes possible to prevent the debris from being reflected by the surface of the stage and from depositing on the silicon substrate.
After a plurality of through-holes is formed in the silicon substrate, the debris within the through-holes can be removed by irradiating again the laser beam into the respective through-holes.
When the through-hole is formed in the silicon substrate, the amount of the laser dross formed on the pattern formation side of the silicon substrate can be reduced if the laser beam 18 is irradiated from the other side of the silicon substrate not having the conductor pattern to form the through-hole.
According to the invention, laser processing is conducted in the condition that the metal plating film is formed on the entire surface of the silicon substrate having the conductor pattern formed thereon. Therefore, the heat dissipation property of the silicon substrate can be improved, and heat generated during laser processing can be quickly dissipated from the silicon substrate. Consequently, possible thermal damage to the conductor pattern due to accumulation of heat in the silicon substrate during laser processing can be eliminated.
Since the metal plating film is peeled after laser processing, the deposit (laser dross, etc) formed in the proximity of the open periphery of the hole such as the through-hole during laser processing can be removed.
The protective film that is stable to the removing solution for removing the deposit deposited on the silicon substrate during laser processing protects the pattern formation surface of the silicon substrate. Therefore, when the silicon substrate is washed with the removing solution, after laser processing, to clean the inner wall of the through-hole or recess formed, possible damage of the conductor pattern on the silicon substrate can be eliminated.
As described above, the invention can improve the heat dissipation property of the silicon substrate during laser processing, can easily clean the laser dross deposited and built up to the portion near the open periphery of the through-hole or recess, formed by laser processing, as well as the debris deposited on the inner wall of the through-hole or recess, and can easily form a clean through-hole or recess through laser processing.